Expanded Windows PowerShell Integration

PowerShell support on Windows has been improved with smarter shell selection and better error handling:

• Shell selection now falls back gracefully: if defaultShell is set to "bash" but Git Bash isn't installed, PowerShell is used instead, and vice versa
• The shell setting description now reads "Defaults to bash (powershell on Windows without Git Bash)"
• Clipboard operations on Windows now use proper argument arrays instead of concatenated strings for safer command execution
• Clear error messages guide users when neither Git Bash nor PowerShell is available:
• "Claude Code on Windows requires either Git for Windows (for bash) or PowerShell"
• Hook errors now suggest "Install Git for Windows, or add shell: powershell to this hook's config"